96 



it will facilitate the progress of the student in in- 

 vestigating the names of rocks ; and as many of 

 them can be rendered sufficiently intelligible. 



Independently of those varieties of character 

 of a minuter kind which are in some measure 

 common to rocks and minerals, the former are 

 also, in many cases, distinguished by certain ex- 

 ternal modes of configuration or disposition, as the 

 latter are by their geometric forms. As these con- 

 stitute an essential part of the natural history of 

 rocks, and are frequently referred to in the follow- 

 ing pages, an enumeration of them is equally re- 

 quired. That enumeration is indeed rendered 

 peculiarly necessary by the geological principle 

 of arrangement which has been adopted. It may 

 perhaps be conceived to belong more properly to 

 a system of geology. Assuredly, it would require 

 to be detailed in every such system ; but the rea- 

 der who shall imagine it rescinded from the pre- 

 sent work, will soon become sensible of the incon- 

 veniences that would arise from its absence. 



The following are the circumstances which it 

 lias been judged proper to select as the general 



